This invention relates generally to electrophotographic copiers, and more particularly to color copiers in which a receiver sheet is sequentially registered with successive color separation images on an image transfer member.
Electrophotography has enjoyed rapid growth as a convenient and efficient means for reproducing original documents. While most electrophotographic copiers produce black-white reproductions, the industry has recently sought to employ similar electrophotographic technology in producing color copies. Two techniques have emerged for using such technology for color copying: (1) simultaneous or (2) sequential exposure of nonoverlapping color separation images on one or more photoconductive members.
An electrophotographic apparatus utilizing simultaneous exposure techniques is shown, for example, in U.S. Pat. No. 3,690,756, issued Sept. 12, 1972 in the name of Smith. In the arrangement of FIG. 1 of the patent, the color separation images are exposed simultaneously on a corresponding number of photoconductive drums. In this arrangement, elements of the apparatus, such as the photoconductive members, are duplicative. Further, the spacing and circumferential size of the photoconductive members must be carefully selected to enable the color separation images to be transferred to the receiver sheet in properly registered superimposed relation. Alternatively, in the arrangment of FIG. 4 of the patent, the color separation images are exposed simultaneously in nonoverlapping relation on a photoconductive web. In this arrangement, the exposure area of the web is substantially larger (e.g., three times as long) than a single image exposure area with a concomitant increase in the size of the apparatus. The larger exposure area increases the complexity of the apparatus to maintain the proper location of the web in the exposure area.
Sequential image exposure is shown, for example, in U.S. Pat. Nos. 3,841,751, issued Oct. 15, 1974 in the name of Draugelis et al, and 4,120,577 issued Oct. 17, 1978 in the name of Watanabe et al. As with simultaneously exposed color separation images located in nonoverlapping relation on a photoconductive web, the receiver sheet must be re-presented to the web to transfer the images in properly registered superimposition. Re-presentation of the receiver sheet is accomplished by tacking the sheet to a roller or a belt which delivers the sheet back to the photoconductive web the number of times corresponding to the number of color separation images. The re-presenting apparatus disclosed in the U.S. Pat. No. 3,841,751 is a transfer roller to which the receiver sheet is clamped. Roller transfer apparatus typically are highly sensitive to environmental conditions and therefore difficult to control. Further, sheet clamping mechanisms are structurally complex, adding significantly to the cost of the apparatus and decreasing its reliability. The re-presenting apparatus disclosed in the U.S. Pat. No. 4,120,577 is a rotating belt which recirculates the receiver sheet. While the belt re-presenting apparatus disclosed in the U.S. Pat. No. 4,120,577 (or the roller apparatus disclosed in the U.S. Pat. No. 3,690,756) uses a more reliable corona charger, mounting of the charger within the re-presenting apparatus is difficult. Additionally, the use of a single charger for the plurality of transfer steps may require additional charger control since the different colored developer materials for developing the color separation images may have different electrical characteristics. Moreover, with a single re-presenting apparatus misalignment of the receiver sheet relative to the apparatus may be magnified on successive passes for image transfer.